The present invention relates generally to network interfacing, and more particularly, to a method for recovering frequency redundant data in a network communications receiver.
The transmission of various types of digital data between computers continues to grow in importance. The predominant method of transmitting such digital data includes coding the digital data into a low frequency baseband data signal and modulating the baseband data signal onto a high frequency carrier signal. The high frequency carrier signal is then transmitted across a network physical transmission medium such as electrical cable, fiber optic, RF, or other medium to a remote computing station.
At the remote computing station, the high frequency carrier signal must be received and demodulated to recover the original baseband signal. In the absence of any distortion across the network medium, the received signal would be identical in phase, amplitude, and frequency to the transmitted carrier and could be demodulated using known mixing techniques to recover the baseband signal. The baseband signal could then be recovered into digital data using known sampling algorithms.
One problem with such networks is that the physical medium and network topology tend to distort the high frequency carrier signal. Branch connections and different lengths of such branches cause reflections of the transmitted signal. Such problems are even more apparent in a network which uses home telephone wiring cables as the network physical medium. The typical wiring of the telephone network is designed for the xe2x80x9cplain old telephone servicexe2x80x9d (POTS) signals in the 0.3-3.4 kilohertz frequency and are not designed for transmission of high frequency carrier signals in a frequency range greater than 1 MHz. The high frequency carrier signal is also distorted by transients in wiring characteristics due to on-hook and off-hook switching and noise pulses of the POTS (e.g. ringing). The high frequency carrier is further distorted by spurious noise caused by electrical devices operating in close proximity to the xe2x80x9ccablexe2x80x9d medium.
Such distortion of frequency, amplitude, and phase of the high frequency carrier signal degrades network performance and tends to impede the design of higher data rate networks. Known techniques for compensating for such distortion and improving the data rate of a network include complex modulation schemes and frequency diversity.
Utilizing a complex modulation scheme such as quadrature amplitude modulation (QAM), both the amplitude and phase of the high frequency carrier are modulated to represent in-phase (I) and quadrature (Q) components of a baseband signal. Referring to FIG. 1, a 4-QAM modulation constellation 10 is shown. In operation, each data symbol is represented by an I-value of+1 or-1 and a Q-value of+1 orxe2x88x921 such that the data symbol can be represented by one of the four states 12(a)-12(d) in constellation 10. Each constellation point 12(a) -12(d) represents a unique combination of carrier amplitude and phase. For example, constellation point 12(a) represents a carrier amplitude of 14 and a carrier phase 16.
FIG. 2 illustrates the utilization of frequency diversity by transmitting the same data in three mutually exclusive sub-spectra 18(a)-18(c) of the transmission band 20. Therefore, if a portion of the band is distorted (e.g. one or more of the sub-spectra 18(a)-18(c)), the data may still be recovered at the receiver from a less distorted portion of the sub-spectra 18(a)-18(c). For example, a data signal modulated onto a 7 MHz carrier utilizing 6 MHz of bandwidth may include three mutually exclusive sub-bands 18(a)-18(c) centered at 5 MHz, 7 MHz and 9 MHz.
One approach to demodulating complex signals is to use filters implemented by digital signal processing (DSP), which provides for a convenient way of varying filter coefficients for each transmission to accommodate carrier distortion as detected in the particular time frame in which the data is being transmitted. Using such approach, an equalizer in the receiver compares the distorted received signal representing a known preamble transmission (prior to the data transmission) to the undistorted waveform of the preamble and determines the appropriate filter coefficients for recovery of the received signal. Such filter coefficients are then used for receiving the data transmission.
In accordance with DSP technology, the high frequency carrier is typically sampled with an A/D converter at a rate that is at least 4 times that of the carrier frequency. Assuming a carrier frequency on the order of 7 MHz, the sampling rate will be on the order of 30 MHz. A problem associated with processing digital samples at such rates to demodulate a complex modulated carrier, and to process mutually exclusive sub-bands of a frequency diverse system, is that very large and costly DSPs would be required.
More specifically, a typical receiver used for demodulating a complex modulated carrier in a noisy environment includes an equalizer and a slicer digitally implemented. The equalizer includes various filters with adaptive filter coefficients for reshaping the noise-distorted signal. The slicer maps the equalized signal to a sequence of constellation coordinates to recover the transmitted data. The slicer also feeds back error data (difference between an equalized coordinate and a defined coordinate) to the equalizer for updating the filter coefficients.
A problem with using such known systems in a frequency diverse environment is that the equalizer may include a 10-15 TAP complex FIR filter to equalize the signal and the error feed back loop in the equalizer may include additional filters of comparable complexity. The multipliers associated with these filters require significant hardware size and complexity. Therefore, simply replicating the equalizer and slicer for each of the sub bands in a frequency diverse environment would result in large and costly DSP circuitry.
Therefore, based on the industry recognized goals for size and cost reduction, what is needed is a receiver system and method for recovering data from a frequency diverse carrier that does not suffer the disadvantages of known systems.
A first aspect of the present invention is to provide a demodulation circuit comprising: a) an A/D converter generating a series of samples representing a frequency diverse modulated carrier including redundant data transmitted in a plurality of sub-spectra; b) a mixer receiving the series of samples and generating frequency shifted I-signal and a frequency shifted Q-signal representing redundant data in a plurality of frequency shifted sub-spectra; c) a filter bank including a plurality of narrow band filters, each centered at one of a plurality of the frequency shifted sub-spectra, each receiving the frequency shifted I-signal and the frequency shifted Q-signal, and each generating a sub-spectrum data signal comprising a sub spectrum I-signal and a sub-spectrum Q-signal; d) a selection circuit determining which one of the sub-spectrum data signals is the strongest sub-spectrum data signal; and e) a receiver circuit generating digital data in response to the strongest sub spectrum data signal.
In the preferred embodiment, the selection circuit operates by comparing a strength value of each sub-spectrum data signal to a strength value of at least one other of the sub-spectrum data signals and selecting the sub spectrum data signal with the greatest strength value to pass to the receiver. The strength value may represent the sum of the strength of a plurality of sample values in the sub-spectrum data signal. The sample values from the first sub-spectrum and the second sub-spectrum may be within a defined time window and the strength value may be equal to the sum of the square of each of an I sample value and a Q sample value occurring within the defined time window.
Further, the selection circuit may select a plurality of defined time windows within a transmission frame and select sub-spectrum with the largest strength value for each of the defined time windows to pass to the receiver to provide the receiver circuitry with the highness strength signal throughout the transmission frame. Further yet, the receiver may include an equalizer and a slicer for recovering quadrature amplitude modulated data from the sub-spectrum data signal. There may be three 2 Mhz sub-spectra centered at 5 Mhz, 7 Mhz, and 9 Mhz respectively.
A second aspect of the present invention is to provide a method of recovering data from a frequency diverse modulated carrier comprising: a) generating a series of digital samples representing redundant data transmitted in a plurality of sub-spectra of a frequency diverse modulated carrier; b) mixing the series of digital samples with a digital sine waveform and a digital cosine waveform to generate a frequency shifted I-signal and a frequency shifted Q-signal representing redundant data in a plurality of frequency shifted sub-spectra; c) filtering the frequency shifted I-signal and the frequency shifted Q-signal with a filter bank including a plurality of narrow band filters, each centered at one of a plurality of the frequency shifted sub-spectra to generate a plurality of sub-spectrum data signals, comprising a sub spectrum I-signal and a sub-spectrum Q-signal, each corresponding to one of the frequency shifted sub-spectra; d) determining which one of the sub-spectrum data signals is the strongest sub-spectrum data signal; and e) recovering digital data in response to the strongest sub spectrum data signal.
The step of determining which one of the sub-spectrum data signals is the strongest sub-spectrum data signal may include comparing a strength value of each sub-spectrum data signal to a strength value of at least one other of the sub-spectrum data signals and selecting the sub spectrum data signal with the greatest strength value. The strength value may represent the sum of the strength of a plurality of sample values in the sub-spectrum data signal. The sample values from the first sub-spectrum and the second sub-spectrum may be within a defined time window and the strength value may be equal to the sum of the square of each of an I sample value and a Q sample value occurring within the defined time window.
The method may further include determining which one of the sub-spectrum data signals is the strongest sub-spectrum data signal for a plurality of defined time windows within a transmission frame and selecting the sub-spectrum with the largest strength value for each of the defined time windows to provide the receiver circuitry with the highness strength signal throughout the transmission frame. Further, the method may include equalizing and slicing the sub-spectrum data signal with the highest strength to recover quadrature amplitude modulated data from the sub-spectrum data signal. There may be three 2 Mhz sub-spectra centered at 5 Mhz, 7 Mhz, and 9 Mhz respectively.